Cooling requirements for internal combustion engines are subject to wide variations depending upon operating conditions. When the engine is relatively cold, little or no cooling is required. Indeed, in very cold operating conditions, cooling may be undesirable. During engine operation, the necessary cooling varies as a function of engine load, and with external conditions such as air temperature and wind or vehicle velocity.
The conventional internal combustion engine is provided with an engine driven fan. The fan can absorb a considerable proportion of the total output power of the engine. For example, in large diesel over-the-road tractors, the fan might require as much as 55 horsepower. With that in mind, and considering the requirements for fuel efficiency, fan drives have been developed which operate only when cooling is needed. Thus, when the engine is operating in normal load and at cruising speeds, there may be adequate air flow through the radiator without the fan, to allow the fan to be declutched from the engine. Conversely, in stop-and-go traffic, under heavy loads, or when parked and idling, the clutch may be engaged to couple the fan to the engine to provide cooling air flow.
If space were not a problem, it would be relatively straightforward to provide a continuously variable relatively reliable clutch mechanism to couple the fan and engine. However, when one appreciates the desires of the truck and engine designers to minimize the space requirements "under-the-hood" and the critical need to efficiently use the under-the-hood space, it will be quickly appreciated that a relatively small envelope is available for the clutch mechanism. The envelope is limited axially by the distance between the radiator and the engine, and it is limited radially, as a practical matter, by the size of sheave which can be accommodated for the pulley driving the fan.
Conventional clutch mechanisms have had their disadvantages. Approaches utilizing dry clutches have typically resulted in on/off operation since the dry clutch could not slip for long without overheating. Inherent in on/off applications is the typical shock load to the drive unit when the drive clutch is engaged. The shock load is not only undesirable from the viewpoint of loading and wear on the mechanism, but is also aesthetically detrimental. When the vehicle is parked, for example, but the engine is running in order to maintain heat or cooling, the fan clutch will typically cycle on and off, creating significant audible disturbance. A further disadvantage of on/off operation is that the system is effectively a coolant temp loop control system. This introduces a response time delay and the clutch mechanism is incapable of dynamically responding to engine conditions to insure that the fan operates at precisely the desired speed and/or to selectively determine fan speed.
One attempt to avoid the problems with dry clutch fan drives has been the attempted use of viscous coupling between the input and output members of the drive unit. Unfortunately, these approaches have also had their drawbacks. First, viscous couplings have poor release capability and no "lock-up" capability so that the drive input and output members may not be driven at the same speed. Moreover, fan drives using viscous couplings have limited horsepower capability, and cannot quickly dissipate heat from the engine. Most viscous coupling designs are slow to engage after sensing heat, and cannot be completely declutched when cooling is not desired.
Wet clutch mechanisms for driving engine fans have also been used. Wet clutch mechanisms, which typically use oil in the engine sump, have been used to provide relatively continuously variable speed, and will not overheat under most conditions by virtue of the oil-bathed clutch mechanism. In engines having relatively wide operating rpm ranges, and therefore wide operating oil pressure ranges, the clutch mechanism must have a relatively large hydraulic piston operating area to reliably operate the clutch at the oil pressure extremes, that is, from relatively low oil pressures at idle to relatively high oil pressures at high engine speeds. Similarly, they have required relatively bulky mechanisms to pump or pressurize the oil in the clutch housing As a result, wet clutches with adequate horsepower for fan drive operation have been relatively large.
In some applications, such as off-the-road vehicles including tractors, loaders, graders and the like, there is adequate room in the engine compartment to tolerate the relatively large clutch mechanisms typically associated with wet clutches. However, in other applications where space requirements are more critical, including, for example, over-the-road tractors, the requirements for aerodynamics, appearance, vehicle size, vehicle weight and the like have all combined to reduce the size of the engine compartment. Thus, the relatively large wet clutches having adequate horsepower for fan operation are less compact than desired and may pose a problem for such applications.